Method and apparatus for treating electrically-conductive surfaces to make them hardor corrosion resistant



April 20, 1965 JOHNSON 3,179,783

METHOD AND APPARATUS FOR TREATING ELEGTRICALLY-CONDUCTIVE SURFACES TO MAKE THEM HARD OR CORROSION RESISTANT Filed June 20, 1962 INVENTOR. 20/1441 0 4. M529 United States Patent METHOD AND APPARATUS FOR TREATlNG ELECTRICALLY-CONDUCTIVE SURFACES TO MAKE THEM HARD 0R CORROSION RESISTANT Ronald L. Johnson, (Iosta Mesa, Calif assignor, by mesne assignments, to Giannini Scientific Corporation, Long Island, N.Y., a corporation of Delaware Filed June 20, 1962, ,Ser. No. 203,761 14 Claims. (Cl. 219--76) This invention relates to a method and apparatus for treating electrically-conductive surfaces to make them hard and/or corrosion resistant. In particular, the meth- 0d and apparatus utilize heat source means including an electrical plasma-jet torch to effect fusion of a thin layer of the surface of a workpiece, and additionally employ means to form a hard or corrosion-resistant integral coating on the surface region thus fused.

A primary object of the present invention is to pro vide an improved method and apparatus for depositing a hard-surfacing or corrosion-resistant material onto a metallic substrate in such manner that the deposited material is rendered integral with the substrate and is not merely mechanically attached thereto.

An additional object is to provide a method and appanatus for surface treating a workpiece in such manner that only a relatively thin region thereof is fused, so that there is no excessive dilution of the deposited hardsurfacing or corrosion-resistant material, and so that relatively thin workpieces may be treated effectively.

A further object is to provide an electrical plasma-jet torch apparatus and method for hard-surfacing a metallic workpiece in a highly effective and efficient manner, without resulting in blasting or blowing of the molten workpiece material due to excessively high-velocity gas flows through the torch.

A further object is to provide a method and apparatus for alloying simultaneously a plurality of different surfacing materials with each other and with the molten surface of an electrically-conductive substrate, thereby achieving important advantages such as greatly reduced costs of the surfacing materials.

Another object is to provide a method and apparatus for surface treating a metallic workpiece in a controlled, effective manner which does not result in oxidation of the workpiece or in other adverse effects, and which is simple, economical and effective in operation.

Another object is to provide a method and apparatus for surface-working and gas-treating a metallic workpiece to render the surface thereof relatively hard or corrosion resistant.

These and other objects and advantages of the invention will be more fully set forth in the following specification and claims, considered in connection with the attached drawing to which they relate.

In the drawing:

FIGURE 1 illustrates, in vertical central section, an electrical plasma-jet torch arranged to deposit one or more hardsurfacing or corrosion-resistant materials on a metallic work-piece, the deposited materials being alloyed with the substrate but only at the interface therebetween so that an integral bond is achieved without substantial dilution of the coating;

FIGURE 2 is a transverse section taken on line 22 of FIGURE 1; and

FIGURE 3 is a view corresponding generally to FIG- URE 1 but illustrating a shielding tube and additional gas source employed between the workpiece and the front electrode of the torch.

Proceeding first to a description of the apparatus shown in FIGURES 1 and 2, an electrical plasma-jet torch 3,179,783 Patented Apr. 20, 1965 10 is illustrated as mounted in position to direct a jet of plasma 11 against a workpiece or substrate 12 formed of metal or any electrically-conductive material. For example, the workpiece may comprise a sheet or plate of mild steel. It is to be understood that the torch 10 may be manually held and moved by an operator. It may also be mounted by suitable means, not shown, in which event means are provided to move the workpiece and torch relative toeach other. The illustrated workpiece is merely illustrative of any one of a wide variety of work-pieces, for example valve components, which may be treated.

Torch 10 is illustrated to comprise a front or nozzle electrode 13 and a back electrode 14 associated with each other by means of a phenolic insulator 16 as well as by an insulating jacket or housing 17. The front electrode 13 may comprise a ring of copper having a nozzle passage 18 formed centrally therethrough. The illustrated nozzle passage is shaped with a conical inlet portion and a cylindrical outlet or downstream portion. Also formed in the front electrode, around nozzle passage 18, is a cool-ant chamber 19.

The back electrode 14 is generally cup shaped, having a thick central stem 21 formed axially thereof and projecting coaxially toward the nozzle passage 18. A suitable elongated insert or arcing portion 22 is mounted axially at the end of the stem 21 and projects into the conical inlet portion of the nozzle passage. In the illustrated arrangement, the downstream portion of insert 22 is generally conical, in conformance with the cone angle of the nozzle passage inlet.

Defined within the rear electrode, between the bottom cup wall and the rear surface of insulator 16 around stem 21, is a coolant chamber 24. Water is fed into such chamber through an inlet conduit 25 and then is passed through a passage 26 in insulator 16 to the aboveind-icated coolant chamber 19 for the nozzle. After thus passing in series through both of the coolant chambers, the water is discharged at 27 to a suitable drain.

The insulator 16 is formed with an internal wall 28 which is a cylindrical or other suitable surface of revolution about the common axis of insert 22 and nozzle passage 18. Such surface, together with the rear face of nozzle electrode 13 and the front face of a rear insulator 29, defines an arc or gas-vortex chamber 31. Gas is introduced tangentially into such chamber from a gas source 32, the inlet conduit being indicated at 33 and best shown in FIGURE 2. Thus, the gas flows vertically in chamber 31 and then passes vortically through nozzle passage 18 around the arcing insert 22. This effects vortex stabilization of an electric are which is maintained between the downstream end of insert 22 and the wall of nozzle passage 18 by means of a suitable source 34 of current.

Current source 34, which is connected by the respective leads 36 and 37 to the back and nozzle electrodes, is adapted to generate a relatively high-current are normally having an order of magnitude on the order of hundreds of amperes. Such are heats the vortically-flowing gas and thus results in formation of the previously-indicated plasma jet 11. Current source 34 will be referred to as the torch current source, and is to be contrasted with the transfer current source which is indicated at 38. The last-mentioned source is connected by means of leads 39 and 40 between nozzle electrode 13 and the substrate or workpiece 12.

The transfer current source 38 is also normally adapted to supply current on the order of hundreds of amperes, and maintains within the external portion of the plasma 11 an electric arc which passes from the front electrode (at the downstream end thereof) to the workpiece. Stated otherwise, the transfer current source employs the 1% plasma jet 11 as a pipe or conduit through which a highcurrent electric arc is generated between the workpiece and the downstream end of nozzle electrode 13.

It is emphasized that both of the current sources 34 and 3% are fully adjustable to vary the magnitudes of the electric currents supplied thereby. The various gas sources and powder sources indicated herein are also fully adjustable to change their flow or feed rates.

Means are provided to supply to the plasma jet 11, which contains the electric are generated by transfer current source 38, a suitable hard-surfacing or corrosionresistant material. Such means is illustrated to comprise a suitable powder source 41, the powder being fed through a conduit 4-2 by means of a carrier gas. The conduit 42 has an end portion 43 which is oriented adjacent and oblique to the plasma jet 11 in order to entrain the particles of powder therein. The powder is thus delivered to the workpiece 12 and melts to form, integrally with the workpiece, a coating which is indicated at 44-.

It is an important feature of the invention that a plurality of powder sources, such as two, three or more, may be employed. A second such source is shown at 56 as connecting to a conduit 57 the discharge end 58 of which is oriented adjacent plasma jet l1, similarly to end 4?. Instead of supplying additional surfacing powder to the jet for purposes to be indicated subsequently, source 56 may be caused to supply flux in liquid or particulate form to aid in shielding the workpiece from oxidation.

A source of cooling gas 46 may be provided and connected to a passage or conduit 47 which is disposed to direct a jet of cooling gas (and/ or shielding gas to prevent oxidation) against the workpiece in the vicinity of the downstream end of the plasma jet 11. Such cooling gas also serves to remove any powder particles which, for any reason, are not melted in the fusion pool formed on the workpiece surface as will be described.

Referring next to FIGURE 3, the apparatus is identical to that described relative to FIGURES 1 and 2 except as will be specifically stated. In the apparatus of FIGURE 3, the powder source 41 is connected directly to a passage 49 formed in the nozzle electrode 13 and communicating radially with nozzle passage 13. Thus, the powder is entrained in the hot gas at a point upstream from the end of the nozzle passage. More than one powder source may be connected to passage 18, as described in co-pending application Serial No. 126,402, filed June 13, 1961, for an Electrical Plasma-Jet Spray Torch and Method, inventors John W. Winzeler and James F. Tucker.

The apparatus of FIGURE 3 also incorporates a tubular shield t which is mounted concentrically around the plasma jet 11 and extends downwardly from the front face of the torch. Such shield has a diameter substantially larger than that of the nozzle passage, being suitably insulated (as by the indicated insulating ring) from the nozzle electrode in order to prevent substantial shorting of the transfer current source 38. A second gas source 51 is provided and is connected through a conduit 52 to the chamber defined within shield 56, it being understood that the lower end of the shield is spaced above the upper workpiece surface in order that gas introduced into the shield may discharge to the ambient atmosphere.

The powder sources 41 and 55, and conduits 42 and 57, described relative to FIGURE 1 may also be employed relative to FIGURE 3, so that the powder is introduced directly into the shield 5i) instead of into the torch nozzle passage.

Methods of the invention In accordance with one of its major aspects, the present method comprises directing the plasma jet from an electrical plasma-jet torch against a workpiece, maintaining an electric are through the plasma jet between the front electrode of the torch and the workpiece and in such manner that a relatively shallow fusion pool is formed in the vicinity of the point of impingement of the jet, and effecting melting of a suitable hard-surfacing or corrosionresistant material or materials in the fusion pool. The various factors should be so selected that the fused layer is only relatively thin, less than one-tenth inch, so that there is little or no dilution of the surfacing medium with substrate material. It is pointed out that the depth of the fused substrate material is preferably much less than one-tenth inch, such as a few thousandths or hundredths of an inch.

In accordance with a second aspect of the present invention, the method comprises introducing a gas into a shield between the front torch electrode and the workpiece in order to cause the workpiece in the vicinity of the fusion pool to be fully blanketed, such gas being adapted to protect the workpiece from oxidation, or to effect hardening or treating thereof. In accordance with another aspect of the invention, no auxiliary material is introduced into the fusion pool or into the jet, the surface treating being instead performed solely by the gas in conjunction with the arc and plasma jet.

Referring particularly to the apparatus shown in FIG- URES 1 and 2, the method may be performed by disposing the lower surface of torch lit) a predetermined distance, such as one to three inches, from the upper surface of workpiece l2. Cooling water is passed through the coolant chambers 19 and 24 by means of the inlet conduit Z5, connecting passage 26, and outlet conduit 27. Gas is introduced tangentially into the are or vortex chamber 31 from the gas source 32. The gas from source 32 is preferably a suitable oxidation-preventing gas such as argon, it being understood that numerous other gases, for example helium, may also be employed.

The vortically-flowing gas is heated, as previously described, by the torch are which is maintained between arcing insert 22 and the wall of nozzle passage 18 due to application of the torch current source 34. Such source may be a DC. source having its negative terminal connected to back electrode 114 and its positive terminal connected to nozzle electrode 13. The resulting hot gas emanates from the torch in the form of the plasma jet 11 and impinges against the predetermined region of workpiece 12.

The transfer current source 38, which may comprise a DC. source having its negative terminal connected to the nozzle and its positive terminal connected to workpiece 12, is then applied to maintain a transfer are through the jet 11 between the nozzle electrode and the work. The current source 38, the current source 34, the gas flow rate, and other factors are so adjusted that a predetermined pool of fused substrate material is formed in the vicinity of the plasma jet llll. Such fusion pool is, as previously indicated, preferably less than one-tenth inch thick.

A hard-surfacing or corrosion-resistant coating is then applied, for example by the powder source 41 which feeds suitable powder (entrained in a gas carrier) through the tube 42 and to the plasma jet 11. The powder particles are heated by the jet l1 and in the above-indicated fusion pool, so that they are melted and form the surface coating 44. Since there is a true alloy region or interface between the molten surface material and the surface of the fusion pool, the coating is made integral with the substrate 12.

The powder from source 41 is entrained in a carrier gas which is oxidation preventing, for example argon. Gas source may be employed to cool and shield the workpiece in the vicinity of jet Ill. Such source 46 may comprise a suitable source of oxidation-preventing gas, such as argon.

The torch N and workpiece 12 are then traversed relat i ve to each other so that the coating 44 is applied to at least a major portion of workpiece surface. The rate of such traverse is so selected that the depth of the fusion pool will be minimized, as previously indicated.

As examples of numerous surfacing materials which may be applied to various substrates, the coating material in source 41 may be stainless steel powder and substrate 12 may be mild steel. The powder may also comprise iron-cobalt based a-lloys, or iron-nickel based alloys, con taining such additives as tungsten, molybdenum or boron. Well-known trade names for certain of such substances include Stellite and Hastalloy. The surfacing material may also comprise tungsten carbide, for example mixed with a minor quantity of cobalt (such as 88% tungsten carbide and 12% cobalt).

The use of specially prepared surfacing alloys or mixtures frequently involves considerable expense, such alloys or mixtures costing several times the price of the pure metals. Thus, as previously stated, it is an important feature of the invention that two or more materials may be alloyed together in the jet 11 and/or in the fusion pool. The materials thus alloyed together are, in turn, caused to be integral (alloyed at the interface) with the substrate. For example, one material may be introduced into the jet from source 56 and another from source 41, as described relative to either of the illustrated embodiments of the invention. Thus, source 41 may supply chromium and source 56 nickel, the rate of nickel feed being made four times the rate of chromium feed to produce an 80%-20% nickel-chromium alloy. Also as stated heretofore, one of the sources 41 and 56 (or another source, not shown) may be employed to blanket the fusion pool with flux in part-iculater liquid form.

As a specific example relative to the embodiment of FIGURES 1 and 2, let it be assumed that it is desired to coat a mild steel sheet 12, one-quarter inch thick, with stainless steel. A torch 10, having a nozzle passage 18 which is inch in diameter, may be arranged sothat the lower surface of electrode 13 is on the order of 2 /2 inches from the workpiece surface. Six standard cubic feet per hour of argon are introduced from gas source 32 tangentially into chamber 31, and current source 34 is applied with its negative terminal connected to back electrode 14 and positive terminal connected to nozzle 13. Source 34 is adapted to maintain a 200 ampere arc in passage 18, the voltage being approximately 22 volts.

Current source 38, connected with its positive terminal to the workpiece 12, is applied so that the transfer are current is 100 amperes at approximately 30 volts. The powder source 41 is then employed to feed No. 303 stainsteel powder, 100 mesh through the tube 42 at a rate on the order of five pounds per hour. Twenty standard cubic feet per hour of argon gas are employed as the carrier. Source 46 is adapted to feed 60 to 70 standard cubic feet per hour of argon, nitrogen, etc., through tube 47 to the vicinity of the fusion pool.

Referring next to FIGURE 3 in particular, the method may be the same as described above except that gas is introduced from source 51 into the shield tube 50. For example, argon, helium or carbon monoxide, at a rate of approximately twenty standard cubic feet per hour, may be introduced from the source 51 into the chamber defined within the shield. Such gas discharges, together with the gas emanating from nozzle passage 18, through the gap between the lower end of shield tube 50 and the surface of workpiece 12. Thus, the fusion pool and surrounding hot portions of the workpiece are maintained in an inert atmosphere.

The apparatus shown in FIGURE 3 is also particularly suitable for arc-working the surface of a workpiece to which a coating has previously been applied by conventional plasma-spray methods (with no transfer current source 38). In such uses of the apparatus, the flow of powder from sources 41 and 56 is normally stopped. The apparatus of FIGURE 3 is also suitable for effecting hardening of a workpiece surface without applying a coating material thereto. In such instances, the flow of powder from source 41 (or other suitable source of material) is again stopped, and the gas source 51 (and/or source 32) is employed to supply predetermined gases to the vicinity of the fusion pool. For example, both of the sources 32 and 51 may be employed to supply nitrogen, which difiuses into the molten steel to form a nitride adapted to harden the workpiece surface. Other gases which may be employed include natural gas, carbon monoxide or carbon dioxide, for example.

Current source 38 may also comprise a pulse source, for example a capacitor or bank thereof adapted to generate a spark discharge through the jet 11 between the nozzle electrode and work 12. Such spark discharge may be repeated numerous times in order to effect spark hardening of the workpiece surface. In other words, the surface is work-hardened by means of the sparks, which serve to move the molecules of workpiece material.

It is emphasized that the application of the transfer current source 38 between the front electrode and the workpiece is greatly superior to the application of a power source between the back electrode 22 and the workpiece. One reason for this is that the application of power between the rear electrode and the workpiece greatly increases the velocity of the jet emanating from the torch, the velocity increase being frequently such that the molten workpiece material is blasted to undesired locations instead of remaining in position for reception of the coating material as desired.

It is to be understood that different current sources 34 and 38, and combinations thereof, may be employed. Thus, for example, the above stated polarities of either or both of the current sources may be the reverse of that indicated above. Furthermore, source 38 may be adapted to supply single or multi-phase alternating current having a frequency of either 60 cycles or various other frequencies. In addition, the current source 38 may be adapted to supply compounded D.C. and A.C., single phase or multi-phase, the D.C. being connected with either polarity. The D.C. may also be compounded with multi-frequency single-phase or multi-phase A.C. Also, any of the above circuits may be employed in combination with inductive energy supplied to the workpiece surface by means of a suitable coil.

To summarize and emphasize certain of the numerous uses of the present invention, number 304 stainless steel may be applied to a mild steel substrate, or any stainless steel may be applied to any structural steel. For hard surfacing, representative coating or surfacing materials include chromium, nickel, cobalt, iron systems with boron or refractory metal additions. The alloying may be effected before the surfacing operation, and/or may be effected during spraying as described relative to sources 41 and 56. Other representative coating materials include various oxides, borides, nitrides and carbides, which may be applied individually or mixed wit-h each other or with other substances before or during spraying.

As an example of one end product, a grinding wheel may be built up in various ways. For example, diamond bort, silicon carbide, titanium diboride, etc., may be incorporated in iron, nickel, chromium, etc., matrices and applied to suitable supporting surfaces.

It is emphasized that the current sources 34 and 38 should be so connected and operated that there are two separate and individual arcs, one from electrode portion 22 to nozzle 13, and the other from nozzle 13 to work 12. There should be no are from electrode portion 22 to work 12. The transfer are which passes through jet 11 is independent of the rear or back electrode, and is substantially entirely external to torch 10. Thus, such are passes from the electrode 13, at a point adjacent the extreme downstream end of passage 18, to the workpiece.

Although the present specification and claims emphasize the treating of surfaces to render them hard or corrosion resistant, the claims are also to be regarded as covering surfacing operations adapted to accomplish various other results. Thus, for example, surfacing may be intended to make the workpiece more emissive, more attractive, etc.

Various embodiments of the present invention, in addition to what has been illustrated and described in detail, may be employed without departing from the scope of the accompanying claims.

I claim:

1. Apparatus for surface treating an electrically-conductive workpiece, which comprises an electrical plasmajet torch adapted to direct a jet of plasma against a workpiece, said torch having a rear electrode and a nozzle electrode, a first current source to maintain an electric are between said rear and nozzle electrodes to effect heating of gas passed through the nozzle passage in said nozzle electrode, a shielding element disposed around said jet of plasma between said nozzle electrode and said workpiece, said shielding element being insulated from said nozzle electrode and being adapted to define a chamber around said jet of plasma, there being a gap between said shielding element and the surface of said workpiece, and means to maintain a transfer are to said workpiece through only the portion of said jet of plasma which is external to said torch.

2. A method of surface treating an electrically-conductive workpiece to impart thereto properties such as increased hardness and corrosion resistance, which comprises directing an electrical plasma jet from an electrical plasma-jet torch towarda region of the surface of an electrically-conductive workpiece, maintaining a transfer are from said workpiece region through only the portion of said jet which is external to said torch, regulating said transfer are in such manner that a relatively thin layer of said workpiece is melted at said region, supplying to said melted layer at said region a material adapted to impart to said workpiece properties such as increased surface hardness and corrosion resistance, and traversing said torch and workpiece relative to each other to treat a major portion of the surface of said workpiece.

3. A method of applying a surfacing material onto an electrically-conductive substrate in such manner that the surfacing material is structurally integral with the substrate, which comprises providing an electrical plasmajet torch having a back electrode and a nozzle electrode and means to feed gas through said nozzle electrode, maintaining a first are between said back electrode and nozzle electrode entirely within said torch to effect heating of said gas to form a jet of plasma, orienting said torch relative to a substrate to cause said plasma jet to impinge against a predetermined surface region of said substrate, connecting a transfer current source to said substrate and to a portion of said torch other than said back electrode to thereby maintain through the external region of said plasma jet a transfer arc, regulating the rate of gas flow through said torch, the power of said first arc, the power from said transfer source, and the rate of torch movement in such manner that only a relatively thin surface region of said substrate is fused, and introducing a surfacing material into the fusion pool thus formed.

4. The invention as claimed in claim 3, in which said ethod comprises introducing said surfacing material into said plasma jet at a region between said nozzle electrode and said substrate.

5. The invention as claimed in claim 3, in which said method further comprises supplying to the region of impingement of said plasma jet against said substrate a cooling or shielding gas.

6. A method of structurally adhering to the surface of an electrically-conductive substrate a hard-surfacing or corrosion-resistant material, which comprises disposing a predetermined distance above said surface a nozzle electrode having a nozzle passage therethrough, disposing a rear electrode on the opposite side of said nozzle electrode from said surface and having an arcing portion extending into said nozzle passage in spaced relationship from the wall thereof, passing an oxidation-preventing gas through said nozzle passage in a direction from said rear electrode toward said surface, maintaining between said areing portion of said rear electrode and said wall of said e) nozzle passage :1 first are adapted to heat said gas flowing through said nozzle passage and to form a plasma jet impinging against said surface, connecting a current source between said nozzle electrode and said substrate to maintain through said plasma jet a transfer arc, regulating said current source to control the power in said transfer are in such manner that a relatively thin fusion pool is formed on said surface in the vicinity of said plasma jet, and introducing a hard or corrosion-resistant surfacing material into said fusion pool for fusing with said substrate, said fusion pool being sufficiently thin that there is relatively little dilution of said surfacing material with substrate material.

7. The invention as claimed in claim 6, in which said method includes introducing said surfacing material into said fusion pool by entraining a surfacing material in powder form into said plasma jet at a point between said rear electrode and said substrate.

8. A method of hardening the surface of an electricallyconductive workpiece, which comprises directing a jet of plasma from an electrical plasma-jet torch to a predetermined region of the surface of said workpiece, maintaining an are through said jet between said torch and said workpiece, regulating the power of said are to effect fusion of a thin layer of the surface of said workpiece, and blanketing the fusion pool thus formed with a gas adapted to diffuse into said fused workpiece material to effect hardening thereof.

9. A method of surface treating an electrically-conductive workpiece to impart thereto properties such as increased hardness and corrosion resistance, which comprises directing a plasma jet from an electrical plasma-jet torch toward a portion of the surface of an electricallyconductive workpiece, maintaining a transfer electric are through the portion of said plasma jet which is external to said torch and by means of an electric circuit including said workpiece, regulating parameters including the current fiow in said transfer arc in such manner that a relatively shallow region of said workpiece portion is melted to form a fusion pool, supplying to said fusion pool at least two distinct materials adapted to impart to said workpiece properties such as increased surface hardness and corrosion resistance, and traversing said torch and workpiece relative to each other to treat a major portion of the surface of said workpiece.

10. The invention as claimed in claim 9, in which said two distinct materials are supplied to said fusion pool by introducing said materials in powder form and from separate sources into said plasma jet.

11. The invention as claimed in claim 10, in which said method further comprises blanketing said fusion pool with a liquid fiux.

12. The invention as claimed in claim 10, in which said method further comprises blanketing said fusion pool with a particulate flux.

13. A method of structurally adhering to the surface of an electrically-conductive substrate a hard-surfacing or corrosion-resistant material, which comprises disposing a predetermined distance above said surface a nozzle electrode having a nozzle passage therethrough, disposing a rear electrode on the opposite side of said nozzle electrode from said surface and having an arcing portion spaced from the wall of said nozzle passage, passing an oxidation-preventing gas through said nozzle passage in a direction from said rear electrode toward said surface, maintaining between said arcing portion of said rear electrode and said wail of said nozzle passage a first arc adapted to heat said gas flowing through said nozzle passage and to form a plasma jet impinging against said surface, connecting a current source between said nozzle electrode and said substrate to maintain through said plasma jet a transfer arc, regulating said current source to control the power in said transfer are in such manner that a relatively thin fusion pool is formed on said surface in the vicinity of said plasma jet, providing a hard or corrosion-resistant surfacing material in particulate form, impinging particles of said surfacing material against said plasma jet in a continuous manner and at a point between said surface and said nozzle electrode, whereby said material is carried by said plasma jet into said fusion pool, and continuously passing an inert gas around said plasma jet and in blanketing relationship over said fusion pool.

14. Apparatus for surface treating an eleetrical y-conductive workpiece, which comprises a nozzle electrode having a nozzle passage therein, a back electrode having an arcing portion coaxial with said nozzle passage, means to eifect flow of gas through said nozzle passage to a workpiece, a first current source connected between said back electrode and said nozzle electrode to maintain an are from said arcing portion of said back electrode to the wall of said nozzle pasasge, said are heating said gas to form plasma, a second current source adapted to maintain a transfer are in said plasma, said second current source having one terminal connected to said workpiece and another terminal connectedto'supply energy to said transfer are independently of said arcing portion of said back electrode, means to supply to said workpiece in the vicinity of said plasma a hard-surfacing or corosion-resistant surface material adapted to coat said workpiece, said surface ma- 10 terial having a composition substantially different from that of said workpiece, said last-named means including means to impinge against said plasma at a point downstream from said nozzle electrode, a hard-surfacing or corrosion-resistant surface material in particulate form, and means to pass continuously an inert gas around said plasma for the purpose of blanketing the surface being treated, said last-named means being independent of said means to efifect flow of gas through said nozzle passage.

References Cited by the Examiner UNITED STATES PATENTS 1,978,316 10/34 Miller 21973 2,806,124 9/57 Gage 219--121 2,841,687 7/58 Richter 21976 2,858,411 10/58 Gage 219 2,906,858 9/59 Morton 219121 2,945,119 7/60 Blackman 219123 2,973,426 2/61 Casey 21975 2,977,457 3/61 Houldcroft 21974 3,145,287 8/64 Siebein et al 21975 3,149,222 9/64 Giannini et a1. 219121 RICHARD M. WOOD, Primary Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No, 3,179,783 April 20, 1965 Ronald L. Johnson It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected belo' Column 9, line 24, for "corosion-resistant" corrosion-resistant Strike out the comma.

read column 10, line 4, after "electrode" Signed and sealed this 28th day of September 1965.

(SEAL) Anest:

ERNEST W. SWIDER EDWARD J. BRENNER Attesting Officer Commissioner of Patents Patent No@ 3,179,783 April 2-0, 1965 Ronald L. Johnson It is hereby certified that err ent requiring correction and that th corrected below.

or appears in the above numbered pate said Letters Patent should read as Column 9, line 24, for corrosion-resistant Strike out the comma,

"corosion-resistant" read column 10, line 4, after "electrode" Signed and sealed this 28th day of September 1965.

(SEAL) Allest:

ERNEST W. SWIDER EDWARD J. BRENNER Attesting Officer Commissioner of Patents 

1. APPRARTUS FOR SURFACE TREATIG AN ELECTRICALLY-CONDUCTIVE WORKPIECE, WHICH COMPRISES AN ELECTRICAL PLASMAJET TORCH ADAPTED TO DIRECT A JET OF PLASMA AGAINST A WORKPIECE, SAID TORCH HAVING A REAR ELECTRODE AND A NOZZLE ELECTRODE, A FIRST CURRENT SOURCE TO MAINTAIN AN ELECTRIC ARC BETWEEN SAID REAR AND NOZZLE ELECTRODES TO EFFECT HEATING OF GAS PASSED THROUGH THE NOZZLE PASSAGE IN SAID NOZZLE ELECTRODE, A SHIELDING ELEMENT DISPOSED AROUND SAID JET OF PLASMA BETWEEN SAID NOZZLE ELECTRODE AND SAID WORKPIECE, SAID SHIELDING ELEMENT BEING INSULATED FROM SAID NOZZLE ELECTRODE AND BEING ADAPTED TO DRFINE A CHAMBER AROUND SAID JET OF PLASMA, THERE BEING A GAP BETWEEN SAID SHIELDING ELEMENT AND THE SURFACE OF SAID WORKPIECE, AND MEANS TO MAINTAIN A TRANSFER ARC TO SAID WORKPIECE THROUGH ONLY THE PORTION OF SAID JET OF PLASMA WHICH IS EXTERNAL TO SAID TORCH. 